1. Field of the Invention
The present invention relates generally to cooking device, and more particularly, to a microwave oven having an infrared ray sensor disposed to sense infrared radiation from food obliquely from the above.
2. Description of the Background Art
Cooking device use an infrared ray sensor as shown in FIG. 15 to sense the temperature of food. The infrared ray sensor converts sensed infrared radiation from the food into electric energy. Referring to FIG. 15, infrared ray sensor 1 includes a base 2, a light receiving portion 3 and an amplifier 4 provided on base 2. Light receiving portion 3 and amplifier 4 are protected in a case 6 having a silicon transparent window 5. Light receiving portion 3 and amplifier 4 are connected to a terminal 7.
Such an infrared sensor used in a microwave oven is a pyroelectric infrared ray sensor formed of monocrystals of lithium tantalate (LiTaO.sub.2). Light receiving portion 3 absorbs infrared rays coming through silicon transparent window 5, and converts the absorbed rays into electric energy. Amplifier 4 is formed of a thick film circuit chip.
The infrared ray sensor responds to those forming intermittent light among incoming infrared rays to provide alternate voltage. Referring to FIG. 16, the microwave oven is provided with a chopper (breaker) 8 having open and closed portions rotating at fixed intervals to have an alternate signal based on the temperature differential between food and chopper 8. The alternate signal is amplified to control the heating temperature using an adder, a comparator and a microcomputer.
Referring to FIG. 16, chopper 8 is rotated by a chopper motor 9 such that the vanes of chopper 8 pass through the light emitting device and light receiving device of a photointerrupter 10 as will be described. A solenoid 11 as will be also described is used to open/close a shutter 12.
FIG. 17 is a view showing the concept of a microwave oven including an infrared ray sensor and the associated portions. The microwave oven has a cavity 17 in which a turn table 18 is provided. Turn table 18 is turned by a pulley 20 through a turn table shaft 19. A cook net 21 is sometimes provided on turn table 18. In FIG. 17, a cup 22a is placed on turn table 18. Microwaves are introduced into cavity 17 from a magnetron 22 through a waveguide 23. Hot air 25 is introduced into cavity 17 through a nozzle 24. Infrared ray sensor 1 is provided at an upper position of cavity 17. Chopper 8 is provided under infrared ray sensor 1. Chopper 8 is rotated by chopper motor 9. In FIG. 17, shutter 12 is provided under chopper 8, and shutter 12 is opened/closed by solenoid 11. Though not shown, such a conventional microwave oven is provided with a dedicated cooling fan for cooling infrared ray sensor 1. Cooling air from the cooling fan is let in in the direction of arrow A, and let out in the direction of arrow B. A beam denoted by reference numeral 25 is infrared radiation from food.
Now, the chopper and the chopper motor will be described further in detail in conjunction with FIG. 18.
In order to convert the temperature of food into an electrical signal using an infrared ray sensor, the dose differential between infrared radiation from food and infrared radiation from a reference object is produced. The chopper is provided for the purpose between the light receiving portion of the infrared ray sensor and incoming infrared rays radiated from food.
Referring to FIG. 18, chopper 18 has three vanes and have vane portions and other portions with no vane provided at equal intervals. Chopper motor 9 is formed of a 24-pole stator having coil windings and a rotor having a permanent magnet, and applies a rotating force to chopper 8. Chopper 8 is fixed to chopper motor 9 by a spring 13, a washer 14, an idle bush 15 and an E ring 16.
Referring to FIG. 19, the solenoid and the shutter will be described in further detail.
Infrared rays from food come into the light receiving portion of the infrared ray sensor through a microwave cutoff pipe 27, and therefore smoke containing oil emitted during the heating operation of the microwave oven comes into the sensor through microwave cutoff pipe 27. Cooling air for the infrared ray sensor coming into the oven through microwave cutoff pipe 27 adversely affects the oven temperature. In order to avoid the effect, referring to FIG. 19, the upper surface of microwave cutoff pipe 27 is closed by shutter 12 operated by solenoid 11 unless the sensor operates. When the sensor operates, solenoid 11 is excited to open shutter 12. In FIG. 19, the portion in dotted line 26 corresponds to the position of shutter 12 during the operation of the sensor. Shutter 12 is opened/closed by solenoid 11 and a shutter spring 28.
Referring to FIG. 20, the photointerrupter will be described further in detail. Referring to FIG. 20, photointerrupter 10 is a photocoupling element formed of a combination of a light emitting device (LED) 29 and a light receiving device (phototransistor) 30. Chopper 8 rotates in the direction of arrow A. When a vane of chopper 8 is between these devices (in the state shown by the oblique lines in FIG. 20), light is cut off and the light receiving device 30 of photointerrupter 10 is turned off. This is serially repeated using the chopper motor to generate a signal having a rectangular waveform at equal intervals. Meanwhile, the waveform generated by the infrared ray sensor is in an alternate form if the food temperature and the chopper temperature are reversed from each other, and therefore the signal of photointerrupter 10 and the signal of the infrared ray sensor are synchronized for comparison. As a result, if the temperature of food is higher than the temperature of the reference object, positive voltage results, and otherwise negative voltage results (which will be further described in Description of the Preferred Embodiments in conjunction with the accompanying drawings).
Referring to FIG. 17, the conventional microwave oven should be provided with infrared ray sensor 1 over cavity 17 and a dedicated cooling fan for cooling infrared ray sensor 1. As a result, a large area is occupied by the microwave oven. In addition, since infrared ray sensor 1 is provided over cavity 17, bits of food placed on turn table 18 bump against infrared ray sensor 1. Furthermore, infrared ray sensor 1 is stained with oil coming up from a food. In addition to the dedicated cooling fan for the infrared ray sensor, the shutter and the solenoid shown in FIG. 19 must be provided, which pushes up the entire cost.